Fiber nonlinearity is and important concerns for large capacity fiber transmission. When we try to increase system capacity, we occupy a wide optical bandwidth and use a high order modulation format, and thus need a high signal-to-noise ratio (SNR) to keep a certain level of bit-error-rate (BER). Consequently, more and more power is launched into the fiber. It is then that the fiber nonlinear effect becomes a significant factor that degrades signal quality.

Scientists have begun to find ways to mitigate fiber nonlinearity. The hint is that the nonlinear noise is highly correlated to signals themselves. Because of this we mitigate nonlinearity by performing special treatment of the signals or conducting post-transmission digital signal processing (DSP) on received signals. The possible mitigation methods include (i) digital back-propagation, (ii) spectral inversion, and (iii) phase-conjugated twin-wave.

Digital back-propagation, as the name suggests, is a DSP method that tries to re-wind the nonlinear channel. In this method, the optical channel is rigorously modelled and the received signals are digitally back propagated through a modelled ‘virtual’ channel. In this way, part of the nonlinearity can be cancelled. The problem associated with this method is that the channel cannot be modelled very accurately due to random parameters during transmission. For instance, the polarization mode dispersion (PMD) is a random parameter that can only be statistically characterized.

Another method, spectral inversion, tries to inverse the spectrum in the mid-distance of fiber transmission, and use the inversed spectrum to propagate the rest half of the distance. By doing so, the nonlinearity accumulated in the first half of the span will be automatically cancelled with those gained in the second half of the span. The method is effective per experimental demonstration. However, the generation of the “inverted spectrum” in the mid-span is rather complicated. Additionally, the application of this method is limited to long haul point-to-point transmission, because otherwise (e.g. transmission in dynamic routed network) the ‘mid-span’ point is hard to identify.

The phase-conjugated twin-wave has similar fundamental principle as the ‘spectral inversion’ method. At the transmitter, signals are polarization multiplexed with one polarization and a phase-conjugated signal against the other. After transmission, special DSP is designed to cancel nonlinearity by overlapping signals from the two polarizations. The nonlinearity cancellation is very effective with not much complexity added to the whole system. However, the method does sacrifice half of the spectral efficiency, since information needs to be duplicated to achieve the ‘twin-wave’. This works has been published in Nature Photonics. For more details, please refer to “Phase-conjugated twin waves for communication beyond the Kerr nonlinearity limit”.

We can find quite a number of presentations in OFC on fibre nonlinear effects and nonlinearity mitigation. Here are a few sessions to go: